1. Field of the Invention
This invention relates to processes for fabricating integrated circuits, particularly processes in which refractory metal films on semiconductor structures are to be etched. Most specifically the invention relates to the etching of tungsten-titanium layers using triethylamine and hydrogen peroxide.
2. Description of the Prior Art
In the manufacture of integrated circuits active and passive electronic devices are formed in a semiconductor substrate and then interconnected with each other, among other ways, by the formation of electrically conductive material on the upper surface of the semiconductor substrate. The regions of electrically conductive material also provide a path for supplying desired electrical signals to these devices, and receiving the resulting output signals.
One now well-known technique for fabricating electrical connections is to utilize layers of tungsten and titanium, or other refractory metals, in conjunction with aluminum layers. The tungsten-titanium provides a barrier to stop plasma etching of overlying layers, is resistant to high temperatures, and prevents overlying layers from dissolving the underlying silicon and changing the electrical characteristics of the semiconductor structure. Also well-known are the formation of metal silicide electrical connections, such as platinum silicide, to regions in the semiconductor structure.
In one commercially available semiconductor device a platinum silicide contact is used to provide an electrical connection to the doped silicon beneath it. An alloy of tungsten and titanium, typically about 10% titanium, is used between the platinum silicide contact and the primary conducting metal layer of aluminum or gold. The tungsten-titanium film provides the necessary adherence to the underlying silicon dioxide, silicon nitride, and platinum silicide materials. In addition, the tungsten-titanium film serves as a diffusion barrier to prevent the overlying aluminum from dissolving into the underlying silicon and shorting junctions or changing the electrical characteristics of the semiconductor structure.
Tungsten-titanium is not etchable in the usual chemical etchants used to define the aluminum conductors. Thus, in a typical semiconductor fabrication process, after etching the primary conductor (usually aluminum), the tungsten-titanium layer is etched in a separate solution. One common prior art technique for etching tungsten-titanium is to use hydrofluoric acid. Unfortunately this has been found detrimental because the hydrofluoric acid also etches aluminum, silicon dioxide, and platinum silicide, adversely affecting the performance, if not destroying, the functionality of the semiconductor device. In addition, if an oxidizing agent is present the hydrofluoric acid will also etch any exposed silicon. Tungsten is normally etched using a base such as potassium hydroxide or sodium hydroxide. These will also etch silicon and aluminum. Thin Film Processes, Vossen and Kern, Academic Press, 1978, pages 474-475.
Another technique which has been used to etch tungsten-titanium is to use a 1:5 mixture of hydrogen peroxide (H.sub.2 O.sub.2) and water heated to about 45.degree. C. The etching of a 2,000 Angstrom thick film in such a solution typically requires several minutes.
Unfortunately the use of a mixture of hydrogen peroxide and water also has undesirable side effects. It has been observed that corrosion of the overlying aluminum conductor can occur in the mixture. Although the precise cause is still unknown, it is presently believed that the platinum silicide/tungsten-titanium/aluminum combination forms an electrochemical cell with the hydrogen peroxide and water solution and results in localized corrosion. This situation is aggravated when the aluminum has been etched in a chlorine-containing plasma because residual surface aluminum chloride and chlorine not only cause localized formation of hydrochloric acid, but are also highly ionic which further supports the electrochemical cell.
We have discovered that the acidity of the plasma reaction product residues may be suppressed by using a mixture of hydrogen peroxide and ammonium hydroxide (NH.sub.4 OH) in the ratio of about 3:1. We have found that this mixture prevents the acid corrosion, and that the room temperature etch rate is an order of magnitude faster than the hydrogen peroxide/water solution. Unfortunately, we have also discovered that a mixture of ammonium hydroxide and hydrogen peroxide itself results in further difficulties. The hydrogen peroxide/water mixture has a low ionic character (on the order of 14K ohms). Because the ammonium hydroxide is a highly dissociated inorganic base, the resistivity of the ammonium hydroxide:hydrogen peroxide etchant is about 4 ohms. When this solution is used it has been found that metallized lines connected to ground taps showed accelerated lateral etching resulting in severe undercutting.
For all of the foregoing reasons, an alternative etching solution is desired for the etching of tungsten-titanium layers, and other refractory metal layers, in the fabrication of integrated circuit structures.